1. Field of the Invention
The present invention relates to a reflection characteristic measuring apparatus for measuring a reflection characteristic of a sample, and more particularly to a reflection characteristic measuring apparatus for measuring a reflection characteristic of samples by serially illuminating the samples with illumination light from an incandescent lamp, and a method for calibrating the reflection characteristic measuring apparatus.
2. Description of the Related Art
There is known a conventional reflection characteristic measuring apparatus, e.g. a spectrocolorimeter for use in printed matter, for serially measuring reflection characteristics of printed patches, as shown in FIG. 12. Specifically, in the conventional reflection characteristic measuring apparatus, a sample 903 is illuminated with a light flux 902a from an incandescent lamp 902 which is driven by a driving circuit 901 in a direction of 45° with respect to a normal direction to the sample 903, and a normal direction component 903n in sample reflected light obtained by the illumination is condensed on an objective lens 904 for guiding to a spectral device 905. The spectral device 905 measures a spectral intensity of the normal direction component 903 in the sample reflected light, and transmits spectral intensity data 905a obtained by the measurement to a computation control device 906. The computation control device 906 converts the spectral intensity data 905a into a spectral reflection characteristic by a well-known method, based on spectral intensity data of a standard sample, which has been measured and stored in advance in the similar manner as described above, and a known spectral reflectance factor of the standard sample.
In the case where an incandescent lamp popularly used in the reflection characteristic measuring apparatus is driven by a constant voltage, as shown in FIG. 10, immediately after the incandescent lamp is turned on, the filament temperature indicated by the reference numeral 921 is increased by a rush current, with the result that the emission intensity is instantaneously increased. Then, the emission intensity is gradually reduced, and the incandescent lamp is transited to a normal state as shown by the reference numeral 922. A change in intensity of an emission light flux i.e. a change in emission intensity resulting from an increase in filament temperature differs depending on a wavelength. The emission intensity is changed within several seconds immediately after the lamp is turned on.
FIG. 11 is a diagram showing an example of a change in emission intensity, standard at a peak value of the emission intensity, with respect to 450 nm wavelength, 550 nm wavelength, and 650 nm wavelength of an incandescent lamp. As shown in FIG. 11, the degree of change in emission intensity is increased, as the wavelength is shifted to a short wavelength band. An influence of the change in emission intensity is corrected, as shown in FIG. 12, by using a reference spectral section i.e. a reference spectral device in a reference optical system 907 of the reflection characteristic measuring apparatus, and by acquiring spectral intensity data 907a of illumination light from the incandescent lamp 902 (see e.g. Japanese Unexamined Patent Publication No. Hei 11-72388).
In a reflection characteristic measuring apparatus without a reference optical system, or a reflection characteristic measuring apparatus designed to acquire merely data on the light amount with use of a light sensor as a reference optical system, high-precision measurement cannot be performed, because the emission intensity i.e. the spectral intensity is changed. On the other hand, if a reflection characteristic measuring apparatus is designed in such a manner that the apparatus is required to wait until the emission intensity is transited to a normal state, in other words, until the driving condition of the incandescent lamp is stabilized, to secure precision, a long measurement time may be required.